This application claims the benefit of International Application Number PCT/EP00/03703, which was published in English on Dec. 28, 2000.
The present invention refers to a compressor of the refrigerant medium of a refrigeration apparatus for home or similar use, namely of the type driven by a variable supply-frequency electric motor.
The importance that is being ascribed also by public authorities to the limitation, ie. reduction in the energy usage of home appliances in general has led to the issuance of a number of Directives, eg. by the European Commission, which, to the purpose of bringing about an overall reduction on the so-called greenhouse effect, practically forbid appliances to be put on the market which use too high an amount of energy to perform the duty they are required to ensure. This applies in particular to compression-type refrigerating appliances, which are largely diffused in all countries worldwide.
In view of assuring compliance with these Directives, proposals are largely known to have been made concerning the use, in a generalized manner, of refrigerant medium compressors driven by an electric motor which, instead of operating intermittently as this is the case in traditional appliances, operates continuously, but at a variable speed. In theory, a motor of such a kind is therefore capable of operating at anyone of the infinite values n comprised between a higher limit ns and a lower limit ni.
In order to constantly ensure an optimum storage condition of the foodstuffs stored in the appliance, the above cited limiting values ns and ni are pre-set in accordance with the thermal load which is generally known to be in a proportion with the difference of the temperature Te of the ambient where the appliance is installed (which, in the case of the climatic class defined as ST, is comprised between +10xc2x0 C. and +38xc2x0 C.) to the temperature T1 of the room or compartment where the foodstuffs are stored (which, in the case of a so-called xe2x80x9ccoolerxe2x80x9d refrigeration appliance, is comprised between +2xc2x0 C. and +8xc2x0 C.). In theory, the ratio of ns to ni should be equal to 19. However, owing to a number of well-known practical difficulties, including the risk of an inadequate lubrication at the low running speeds of the compressor motor, said ratio ns ni between the speed limits of the motor of the compressor actually amounts to approx. 3. The preservation of the stored foodstuffs is therefore ensured in an optimum manner only under just a few ones of the actual operating conditions of the appliance, which in fact may deviate even considerably from reality.
A second major drawback deriving from the use of a compressor driven by a variable-speed motor lies in the fact that this requires the refrigeration appliance to make use of a special thermostatic temperature control system that differs both as far as the hardware and the software are concerned from the systems currently used in traditional refrigeration appliances, in which the motor of the compressor operates intermittently at a single speed. A manufacturer of refrigeration appliances is thus forced to face a time-consuming and expensive job of redesigning and testing each single appliance model included in his manufacturing range.
WO-A-98/15790 discloses a speed control of a compressor which is based on a simple ON/OFF signal issued by a thermostat located in the surroundings to be cooled. The method according to said patent is characterised in that the starting speed of the compressor in a following ON period is reduced in relation to the final speed in the previous ON period. Therefore, there is only one parameter (compressor speed) that is measured and compared in two different stages.
EP-A-0 583 560 discloses a refrigerator provided with a compressor driven by an electric 3-phase motor connected with a mono-phase electric line. The control is made on the supply of electric current to the motor by means of a frequency converter of a mono-three phase type. So, the xe2x80x9cduty-cyclexe2x80x9d of the compressor, which is the more significant parameter for an efficient control, is not taken into consideration.
U.S. Pat. No. 4,831,836 discloses a motor compressor for air conditioners, wherein the speed is regulated through a frequency control and means for detecting a difference between a room temperature and a predetermined temperature. In a first solution, the parameter detected is a value of the motor electric current, which is used to vary the frequency of the same motor. In a second embodiment, a pressure within the compressor is detected to control the current of the motor. In both cases, the control is based on the detection of only one parameter and there is the need to employ a specific additional sensor (current or pressure sensor).
U.S. Pat. No. 4,407,139 discloses a method for controlling an air conditioning system, said method being based on determining a frequency set value in accordance with a deviation of a detected temperature of the air to be conditioned with reference to a set temperature. Even in this case there is only one parameter detected and said parameter is not the xe2x80x9cduty cyclexe2x80x9d of the compressor nor the time of functioning the same.
U.S. Pat. No. 5,410,230 discloses a centralised system for heating, conditioning and ventilating a space, which comprises a plurality of motors to drive the various components of the system. Means responsive to the temperature of the air in the space generates a temperature signal having a cycling parameter which corresponds to the temperature of the air in the space as it rises and falls. Therefore, the control is of a continuous type, always carried out on the basis of only one parameter.
A different type of compressor, disclosed in the publication EP-A-0 490 089, is driven by a motor that is capable of operating selectively at a discrete number of speeds (two or, at most, three) and has a volume displacement capacity that is relatively increased with respect to the one of equivalent compressors driven by a single-speed motor. The first speed n1 of the motor, which may be in the order of magnitude of 2,000 rpm, is used in the operating periods of the appliance in which the need or opportunity arises for the energy usage of the same appliance to be minimized (ie. the so-called xe2x80x9cpreservation periodsxe2x80x9d); the second speed n2 of the motor, which may be in the order of 2,800 rpm, and the possibly provided third speed nT, which may amount to something as 3,200 rpm, are on the contrary used when the appliance needs to ensure a particularly sustained refrigeration capacity, such as for instance in such transient periods as a starting after a prolonged pause, or is working under particularly heavy or demanding operating conditions. Switching over from a speed to another one is performed, in the cases in which the motor is of the brushless type, by means of a device adapted to control the supply frequency.
It is a main purpose of the present invention to provide a compressor of refrigerant medium of this second kind, whose electric driving motor is capable of switching over in an automatic manner from a supply frequency to the other one to the purpose of minimizing, under any operating condition, the energy usage of the appliance.
A further purpose of the present invention is to enable the manufacturer to do away with the need of going through a function redesign of his range of refrigeration appliances, thanks to the fact that, at least at the lowest speed n1 thereof, the driving motor of the compressor keeps operating intermittently. Owing to the fact that the present invention does not actually require any modification to be made on the thermostatic temperature control associated to the food storage compartment of the refrigeration appliance in which the compressor is installed, the same system may thus be come down even to a simple, well-known thermostat of the fluid-expansion type.
According to the present invention, these and further aims are reached in a control system having the characteristics as recited in the appended claims.